It doesn't sound much like the forecasts we hear on Earth,
which feature rain and the daily pollen count. On Mars -- a world
that's desert-dry, Antarctic-cold, and possibly lifeless -- human
colonists will have a different set of weather concerns.

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The Red Planet is substantially exposed to the harshest
elements of space weather. Unlike Earth, which sits inside
a protective magnetic bubble called the magnetosphere,
Mars does not have a global magnetic field to shield it from
solar flares and cosmic rays. Scientists aren't sure why, but
Mars' internal magnetic dynamo turned off about 4 billion years
ago. After that, the solar wind gradually eroded
the martian atmosphere until, today, it is less than 1% as thick
as Earth's.

No global magnetic field and a very thin atmosphere -- those
are the two factors that render Mars vulnerable to space radiation.

Does such exposure mean Mars is lifeless?
Not necessarily, say scientists. Indigenous life forms could
be radiation resistant, like the terrestrial microbe Deinococcus
radiodurans. Tiny Martians might
also live in rocks or soil, substances that provide natural protection
against radiation.

Nor is Mars necessarily uninhabitable
for humans. If we learn how to shelter ourselves from the planet's
unique brand of weather, humans can explore and perhaps even
live on Mars. That's why NASA is sending a radiation monitor
to the Red Planet ... to find out how much protection we humans
might require.

MARIE,
the Mars Radiation Environment Experiment, blasted off April
7th with the 2001 Mars Odyssey spacecraft. MARIE is one of three
scientific instruments on board -- the other two will search
for signs of water and interesting minerals on Mars. If all goes
as planned, MARIE (along with the rest of Odyssey) will arrive
in October and spend at least two years circling the Red Planet.

Above: MARIE, which spans less than 12 inches down its
longest side, weighs 7.3 pounds and uses 7 watts of power.

"MARIE can detect charged particles
-- electrons, protons, and atomic nuclei -- with energies between
15 MeV and 500 MeV," says Gautam Badhwar, the experiment's
principal investigator at the Johnson Space Center. "There
have never been any measurements of this kind from Mars orbit,"
he added. (Note: 1 MeV equals one million electron
volts.)

Space radiation can be electromagnetic, like x-rays and gamma-rays,
or particulate, like protons and electrons. Particulate radiation
poses the greater threat to humans.

Most charged particles in our solar system come from two sources:
solar flares, which produce a rain of dangerous protons, and
distant supernova explosions, which accelerate atomic nuclei
--called "cosmic rays"-- to nearly light speed.

"Both
can be hazardous, but from the standpoint of crew health, solar
flares are the greater concern," says Badhwar. Solar flares
produce particles with relatively low energies (~70 MeV). "Such
protons lose energy in tissue at a much higher rate than faster-moving
particles like cosmic rays" he added. Cosmic ray nuclei,
carrying typically 300 to 500 MeV per nucleon, zip through the
human body so quickly there's not enough time to dump their energy
into the surrounding tissue.

Solar protons passing through humans ionize molecules along
their tracks. "The ionization creates free radicals,"
explains Badhwar, "which can be very damaging." Sometimes
protons will modify or even break DNA strands within cells. If
the cell survives it can become cancerous -- a long-term health
risk of radiation exposure.

Mars' thin atmosphere does little to protect the planet from
energetic protons. The air density at martian "sea level"
is roughly equivalent to that of Earth's atmosphere at 70,000
feet altitude! Fortunately, astronauts can find the protection
they need indoors; shelter walls made of lightweight
materials provide adequate shielding. But future explorers
won't want to spend all their time inside shelters. They'll need
to know how to handle radiation levels outdoors in the "martian
wilderness" -- an environment MARIE will probe from Mars
orbit.

Above: Explorers on Mars won't want to stay inside all
the time. Artist Paul Hudson created this scene depicting humans
striding across the Red Planet. [click to enlarge]

Although MARIE won't reach Mars for another six months, the
instrument is already hard at work.

By monitoring radiation levels during Odyssey's cruise phase,
Badhwar and colleagues will discover what sorts of hazards await
travelers in transit from Earth to Mars.

Radiation hazards ... tissue damage ... broken DNA. Space
sounds like a dangerous place! Nevertheless, MARIE is an optimistic
experiment. Its underlying assumption is that humans will eventually
cross the divide between our planet and Mars. Thanks to MARIE
and future experiments like it, Mars explorers will know how
to survive and prosper when they get there.

MARIE
-- The Mars Radiation Environment Experiment is designed to characterize
aspects of the radiation environment both on the way to Mars
and in the Martian orbit.

Human
Exploration and Development of Space -- The goal of NASA's Human Exploration and Development
of Space (HEDS) Enterprise is to open the space frontier by exploring,
using and enabling the development of space

The Accidental Space Tourist -- Science@NASA article: Deinococcus radiodurans beats most
of the constraints for survival of life on Mars - radiation,
cold, vacuum, dormancy, oxidative damage, and other factors.

The Science and Technology Directorate at NASA's
Marshall Space Flight Center
sponsors the Science@NASA web sites. The mission of Science@NASA is to
help the public understand how exciting NASA research is and to help
NASA scientists fulfill their outreach responsibilities.